1. Field
The technical field of this disclosure relates to a semiconductor laser in which a lower cladding layer, an active layer and an upper cladding layer are formed in this order on a semiconductor substrate, and an electronic device in which such a semiconductor laser is applied.
2. Description of the Related Art
Semiconductor lasers are proposed in which a lower cladding layer, an active layer, and an upper cladding layer are formed in this order on a semiconductor substrate. Such semiconductor lasers are applied in electronic devices that perform recording by writing optical data to an optical disk (an optical disk for writing).
Acceleration in the write speed of optical disks has been accompanied by demands for increased optical output relative to conventional semiconductor lasers. Also, there is a trend in recent years towards practical use of optical disks that have two recording layers in order to enlarge the capacity of optical disks, and as a result there are further demands for increased output of semiconductor lasers.
However, increased output of semiconductor lasers is accompanied by the following sorts of problems. The first problem is that an increase in optical output from a light-exiting end face of the semiconductor laser is accompanied by deterioration of the light-exiting end face. The second problem is a deterioration in properties when power is turned on, due to driving current (operating current) that is increased in order to increase the optical output, i.e., there is a reduction in reliability. It is necessary to address these problems in order to apply increased optical output to an optical disk.
As technology that prevents deterioration of a light-exiting end face due to increased optical output, it is known to be effective to form a region in the light-exiting end face where a band gap is enlarged, i.e., a so-called window region (for example, see JP 2003-124569A).
However, the deterioration phenomenon that accompanies increased driving current (operating current) occurs even when end face deterioration has been prevented by forming a window region. That is, even in a state in which optical deterioration does not occur at the end face, increased driving current causes deterioration in the increase in the driving current and deterioration in the optical output, due to crystal deterioration inside the semiconductor laser. Also, it is has been confirmed through testing that under a condition that deterioration does not occur at the end face, operating current density when driving has an effect on reliability.
Accordingly, in order to insure adequate reliability even when optical output has been increased, it is necessary that the operating current density is not increased. In order to prevent reduced reliability due to increased driving current when optical output has been increased, it is effective to increase the length of a resonator. That is, when increasing the driving current of a semiconductor laser, by increasing the length of a resonator it is possible to reduce a current value per unit area, i.e., it is possible to reduce the operating current density.
However, in the semiconductor laser, when the resonator is lengthened, a differential efficiency η (slope of optical output properties to operating current: W/A) decreases. Accordingly, with the differential efficiency η remaining low, it is necessary to increase the operating current in order to realize a predetermined optical output, and as a result there is the problem that the operating current density cannot be adequately reduced.
Also, semiconductor lasers in which an active layer is sandwiched by upper and lower cladding layers are disclosed in JP 2005-101440A and JP 2006-128405A.
The inventors of the present application, in the course of performing various testing, found that as a measure for improving the differential efficiency η, it is effective to reduce the concentration (dopant concentration, or carrier concentration) of a dopant introduced to the lower cladding layer as an impurity. That is, when the operating current density is reduced by increasing the length of the resonator, by reducing the dopant concentration of a lower cladding layer configured for example with n-type conductivity as a first conductivity type, a decrease in the differential efficiency η is prevented, so an increase in the operating current density is suppressed, and thus it is possible to prevent a decrease in reliability.
Also, because the reduction in the dopant concentration of the lower cladding layer increases resistance, it is necessary to increase operating voltage. For example, in an optical pickup (an optical drive such as an optical disk apparatus) that uses a semiconductor laser, a laser driver IC is used in order to drive the semiconductor laser. In the laser driver IC, there is a maximum current rating in consideration of IC heat loss and the like. Also, there is a rating for a maximum rated voltage of laser driving that is determined from the power source voltage specification inside the optical drive and a voltage drop inside the laser driver IC. Accordingly, in order to apply a semiconductor laser in an optical drive (laser driver IC), it is necessary to not exceed the maximum rated current and the maximum rated voltage of the laser driver IC, and it is necessary to suppress an increase in the operating voltage of the semiconductor laser.